A parallel-plate type thin-film forming sputtering system having a metallic sputtering target (cathode) and a substrate arranged parallel to each other within a vacuum chamber has been often used for forming a thin film. In this type of system, an argon gas or similar inert gas is introduced into the vacuum chamber, and a direct-current voltage or radio-frequency voltage is applied to the sputtering target to generate plasma within the vacuum chamber. The target is sputtered by the ions in the plasma so as to form a desired thin film on the surface of the substrate.
An example of the thin-film forming sputtering system capable of forming a thin film at a high rate is a magnetron sputtering system (refer to Non-Patent Document 1). The magnetron sputtering system has an electromagnet or permanent magnet placed behind the target to generate a magnetic field parallel to the surface of the target. This magnetic field is combined with an electric field generated by the direct-current voltage or radio-frequency voltage applied to the target, to produce a cycloidal motion or trochoidal motion of electrons. (These motions are hereinafter collectively referred to as the “cycloid-trochoidal motion.”) This motion of electrons is utilized to generate plasma in a localized form near the surface of the target and thereby increase the plasma density on the surface of the target so that the target will be efficiently sputtered. As compared to the systems that utilize no magnetic field, the magnetron sputtering system has various advantages, such as a higher film-formation rate, stronger adhesion of the film, and less damage to the substrate by virtue of the smaller increase in the temperature of the substrate.
Patent Document 1 discloses a magnetron sputtering system in which a radio-frequency coil is used to create plasma within a space through which the particles sputtered from the target pass to reach the surface of the substrate. While passing through this plasma, the sputtered particles are ionized, and the resulting ions are pulled toward the substrate due to the effect of the aforementioned electric field. Thus, the film is efficiently formed at a high rate.
However, even the aforementioned conventional magnetron sputtering systems cannot sufficiently increase the plasma density in the vicinity of the surface of the target to achieve an adequately high sputtering rate.
The film-formation rate can be improved to some extent by strengthening the electric field applied to the target (the target bias). However, this operation unfavorably increases the damage to the substrate (the plasma damage) caused by the ions impinging on the substrate after colliding with the target with high energy and being recoiled toward the substrate.
Furthermore, in the case of a reactive sputtering which is performed when a thin oxide film needs to be formed, an oxide coating is formed on the surface of the target due to the reaction with oxygen, causing the electrical charge-up of the surface of the target, which relaxes the electric field on the target's surface and eventually lowers the plasma density. As a result, the film formation rate considerably decreases. Thus, with the conventional thin-film forming sputtering systems, it is difficult to form a thin oxide film at a high rate.
Meanwhile, an inductively-coupled sputtering system using radio-frequency antennae has been recently under study. Patent Document 2 discloses an inductively-coupled sputtering system having a vacuum chamber containing two U-shaped radio-frequency antennae with a target surrounding these antennae. According to this document, the inductively coupled sputtering system can be used to create silicon dots (silicon nanoparticles) having an extremely small diameter (16 nm in one embodiment) on a substrate. In this system, inductively-coupled plasma is formed by the radio-frequency antennae, and this plasma has a high density in the vicinity of the radio-frequency antennae. However, the technique described in Patent Document 2 is not aimed at high-speed film formation. Actually, by this technique, it is impossible to intensively increase the plasma density on the surface of the target since the plasma diffuses rom the radio-frequency antennae in all directions, so that the film-formation rate cannot be sufficiently high. For these reasons, the inductively-coupled sputtering system described in Patent Document 2 which is capable of creating extremely small structures (such as the silicon nanoparticles), is impractical for the creation of relatively large structures, such as a thin film having a thickness on the order of micrometers.